Recently, there has been a social movement of restricting exhaust gases, including greenhouse gases, from the perspective of global warming prevention. The response to such a movement has been especially notable in the automobile industry, and the development of so-called eco-cars, such as hybrid cars and electric vehicles, has been promoted. In particular, there has been an active technological development in commercializing electric vehicles using batteries as power sources.
For electric vehicles, in addition to reduction in the battery size and motor size, size reduction in the electronic devices controlling the batteries and motors is a technical problem. There are other problems in reducing the size of electronic devices. That is, to reduce the size of an electronic device, the electronic circuits therein must be integrated. An increase in the electronic circuit density per unit area causes an increase in the amount of heat generation (current flowing through the circuit) per unit area of the electronic circuit, and thus there is a problem in that normal operation of the electronic circuit is inhibited due to an increase in the amount of heat generation. Hence, size reduction of electronic devices and cooling of such electronic devices are problems that must be solve simultaneously.
To solve such two problems, a technical proposition focusing on a motor driver of an in-wheel motor has been made. The corresponding document aims for solving the technical problems described above by changing the electronic circuit provided for the motor driver. That is, the motor driver includes voltage changing means for changing the motor driving voltage. Specifically, the voltage changing means decreases the motor driving voltage in a low-speed rotation range of the motor and increase it in a high-speed rotation range to prevent an excessive current from being unnecessarily applied to the motor driver during low-speed rotation and to reduce the amount of heat generation in the motor driver.